The present invention relates generally to a mold changing system for a thermo-mold continuous casting, which can significantly reduce the mold changing time and thus increase the yield. The system also can facilitate the production of a metal casting having a unidirectional solidified structure and an excellent surface quality.
By conventional continuous casting methods, the casting has a polycrystalline structure, a rough surface, and sometimes local surface cracks. These defects have to be removed before the post process such as the wire drawing or rolling process, generally by a paring method. If the cracks are too deep, however, they will be unable to be removed, and even cannot be machined. In such a situation, the amount of rejected products is thus increased. In order to alleviate these defects, the Japanese Patent Publication No. 55-46265 proposed an operation of continuous casting (OCC) by using a heating mold. In this operation, a graphite mold is heated by a heater surrounding the graphite mold to maintain its inner temperature above the melting point of the cast metal. The mold is supplied with a molten metal from one end thereof. The cast piece is drawn from the other end of the mold by a draw bar, and then is cooled directly by water. The formed cast piece is thus in the form of columnar crystal or single crystal grown along the axial direction thereof. The mold, typically, must have the following properties of being: 1. heat-resistant; 2. declined to react with the metal liquid, and dampproof; 3. heat impact-resistant; and 4. good heat conductivity. In casting copper or aluminum, the graphite is a very excellent mold material because it has all properties described above and a low cost, and can easily be machined. The surface of the graphite mold, however, easily suffers from oxidization at a high temperature, i.e. over 400.degree. C. This will affect the surface quality of the cast piece, and shorten the service life of the mold. When the oxidization of the graphite mold is severe, the graphite mold has to be changed to maintain the quality of the cast piece. The changing of the mold, however, is very time-consuming. For example, in the thermo-mold continuous casting apparatus disclosed in the Japanese Patent Publication No. 63-52753, the crucible of the casting furnace is connected to the heated mold at one side thereof. When it is desired change the mold, the changing task can be performed only after the casting furnace and heating mold are cooled to the room temperature because the mold cavity still has a residual metal.
Referring to FIGS. 1, 2 and 3, FIG. 1 shows a flow chart of conventional mold changing procedure, FIG. 2 shows a conventional thermo-mold continuous casting apparatus, and FIG. 3 shows a thermo-mold assembly used in the thermo-mold continuous casting apparatus. For a clear understanding, the conventional thermo-mold continuous casting apparatus of FIG. 2 is described briefly here. The apparatus includes a thermo-mold chamber 8, a casting furnace 15, and a heat insulation wall 20 located between the thermo-mold chamber 8 and the casting furnace 15. The casting furnace 15 includes a base 12, a cover 13, a control switch 14, and a casting crucible 30. The casting crucible 30 has a base 18, and a surrounding heater 19. The material bar 17 can be fed into the casting crucible 30 through a feeding wheel pair 16. The thermo-mold chamber 8 includes a front surrounding heater 4, a rear surrounding heater 5, a mold supporting frame 6, a chamber cover 7, an outer wall 9, and a bracket 11. In the thermo-mold chamber 8, a mold assembly consisted of a front mold 1, a middle mold 2 and a rear mold 3 is mounted.
The general reason to change the mold is that the inner wall of the front mold 1 has been oxidized, or that the molten metal broke out and then blocked the outlet 31 of the front mold 1. In either situation, although it seems that only the front mold 1 needs to be changed, this is not practical :fin the conventional system. In the conventional mold assembly, the inner channels of front, middle, and rear molds 1, 2, and 3 directly connect together. Thus, when the control switch 14 is turned off to close the flow channel, and front and rear heaters 4 and 5 in chamber 8 are also turned off, the residual molten metal in the inner channels of front, middle, and rear molds 1, 2, and 3 will become getting solidified as a continuous bar or rod 22, as shown in FIG. 3. In such a situation, merely changing front mold 1 is not possible, that is to say that the entire mold assembly has to be changed altogether. To change the entire mold assembly, heater 19 for casting crucible 30, in addition to heaters 4 and 5 for the mold assembly, has to be turned off also. Then, the casting furnace 15 and the thermo-mold chamber 8 have to be cooled down to near room temperature to facilitate the detachment of the mold supporting frame 6 and the entire mold assembly, and to prevent the casting graphite crucible 30 and the graphite mold from oxidization.
This conventional mold changing procedure is summarized in FIG. 1. Firstly, the power of the casting crucible 30 and heating molds 1, 2, and 3 is shut off for about 12-16 hours, as shown in the block 70, in order to let them be cooled to room temperature, as shown in the block 72. After about 0.5 hour, the mold assembly is detached, and a new mold assembly is installed with realignment, as shown in the block 74. This will take about 2 hours. The casting crucible 30 and new mold assembly will be re-heated for about 2 hours, as shown in the block 76, and then the casting can begin again, as shown in the block 78. It can be clearly appreciated that conventional mold changing procedure is very time-consuming, and wastes very much energy.